Ethane extracted from natural gas is an important source of feedstock for ethylene production via industrial scale cracking processes.
The recovery of natural gas liquids (NGLs), such as ethane, from natural gas typically involves absorptive or cryogenic separation methods.
In absorptive separation, natural gas is brought into contact with a “lean” absorption oil which picks up NGLs by extractive absorption of like hydrocarbon compounds. The resulting “rich” absorption oil contains propane, butane and heavier hydrocarbons, while leaving lighter ethane fractions in the natural gas. In a typical process, heating the rich oil to temperatures above the boiling point of the C3+ NGLs allows recovery of approximately 75% of the butane fraction and 85-90% of the pentane and heavier hydrocarbons fraction from a natural gas stream. Although alternative oil absorption methods which use refrigerated absorption oil allow for the recovery of some of the ethane, they are still less effective than cryogenic methods.
Cryogenic methods allow for as much as 90% or more of the ethane present in a natural gas stream to be recovered. Cryogenic processes generally involve cooling a natural gas stream to temperatures below around −120° Fahrenheit. These low temperature requirements have high associated energy consumption costs. For example, one way to lower the temperature of a natural gas stream is to use a turbo expansion process. In this process, refrigerants are used to cool the natural gas stream, followed by rapid gas expansion by an expansion turbine. Expanding the cooled gas produces a rapid temperature drop which condenses out NGLs, including ethane, while methane is left in the gas phase. Subsequently, the gaseous methane effluent must be recompressed to pipeline pressures, requiring further energy input.
In light of the poor efficiency for absorptive processes and the high cost associated with cryogenic methods, there has been interest in alternative technologies for removing NGLs from natural gas. One promising alternative is the use of adsorptive materials to selectively strip paraffinic hydrocarbons from natural gas by selective adsorption. It would be especially useful for the ethane derivatives industry if adsorption methods could separate methane and ethane fractions at high pressure, such as at natural gas pipeline pressures which are from about 200 psia to about 1500 psia. Such methods could augment or replace traditional high cost straddle plant technology which employs cryogenic separation to remove natural gas liquids such as ethane from natural gas streams and pipelines.
International Patent Application No. WO 80/02558 discloses that molecular sieves (4 Angstrom and 10 Angstrom) can be used to selectively adsorb ethane from a mixture containing methane and ethane. However, molecular sieves also adsorb high amounts of methane which can limit application to commercial adsorption/desorption swing processes.
Zeolites 5A and 13× have also been examined as potential materials for light paraffin gas separations. These zeolite materials preferentially adsorb ethane over methane from a binary mixture of the same, but do so with lackluster selectivity (see Loughlin, K. F.; Hasanain, M. A; and Abdul-Rehman, H. B. in Ind. Eng. Chem. Res. 1990, v29, p1535-1546). In a similar work, a silicalite material (Linde S-115) was shown to selectively adsorb ethane and higher paraffinic hydrocarbons over methane (see Abdul-Rehman, H. B.; Hasanain, M. A.; and Loughlin, K. F. in Ind. Eng. Chem. Res. 1990, v29, p1525-1535).
U.S. Pat. Nos. 5,013,334 and 5,171,333 both disclose the use of faujasite type zeolitic aluminosilicate materials in methane gas purification. A pressure swing adsorption process is disclosed in which ethane is selectively adsorbed from a mixture of methane and ethane. The faujasite type zeolites could be chosen from either X or Y type materials.
U.S. Pat. No. 5,840,099 discloses the use of activated alumina, zinc oxide or magnesium oxide for the selective uptake of water, CO2, ethane and C3+ paraffins from natural gas streams.
In light of the potential value of adsorptive methods for ethane sequestration or methane purification processes, alternative materials having high adsorption selectivity are desirable, especially where selectivity and performance may be readily tuned through facile material modification.
One such possibility is to use metal organic frameworks. U.S. Pat. Appl. No. 2009/0216059, shows that “zeolitic” imidazolate framework materials are able to selectively adsorb ethane and higher hydrocarbons (C3+) from a feedstream containing the same mixed with methane.
Another interesting possibility is to use the EXS titanosilicate materials developed by Engelhard Corporation. EXS materials have octahedrally coordinated active sites in their crystal structure and are different from other types of zeolites. EXS zeolites also contain electrostatic units which are different from the charged units in conventional tetrahedrally coordinated aluminosilicate zeolites.
Members of the EXS family comprise all materials based on the structure of ETS-10, including ETS-10 (see U.S. Pat. No. 5,011,591), ETas-10 (see U.S. Pat. No. 5,244,650) and other framework substituted derivatives (see U.S. Pat. No. 5,208,006), as well as ETS-4 (see U.S. Pat. No. 4,938,939) and CTS-1 (see U.S. Pat. No. 6,517,611). Importantly, these materials can have their adsorptive selectivity and behavior radically altered through structural or ionic modification (see for example, CA Pat. Appl. No. 2,618,267). Hence, the performance of these materials can be finely tuned to suit a particular adsorptive application.
EST-4 and CTS-1 are reduced pore titanosilicates, dubbed Molecular Gate™ materials and are available from Engelhard. ETS-4 and CTS-1 have been used to remove polar components (e.g. CO2) and nitrogen from natural gas streams. See for example, U.S. Pat. Nos. 6,197,092; 6,315,817; 6,444,012; 6,497,750; 6,610,124 and 7,314,503. As further shown in U.S. Pat. Nos. 6,610,124; 7,396,388; 7,442,233 and US Pat. Appl. No. 2006/0191410, Molecular Gate materials also selectively remove heavy hydrocarbons (i.e. C3+ paraffinic hydrocarbons) from a natural gas stream while leaving methane and ethane components in the stream. Hence, ETS-4 and CTS-1 materials are unsuitable for methane/ethane adsorptive separation processes.
In contrast, ETS-10 has been shown to be selective for ethane adsorption over methane adsorption (see: Al-Baghli, N. A., Loughlin, K. F. Journal of Chemical and Engineering Data, 2005, v50, p. 843-848 and Al-Baghli, N. A., Loughlin, K. F. Journal of Chemical and Engineering Data, 2006, v51, 248-254). Engelhard Titanosilicate-10 (ETS-10) is a large-pored, mixed octahedral/tetrahedral titanium silicate molecular sieve with a framework composed of a three-dimensional network of interconnecting channels and cavities (see: U.S. Pat. No. 5,011,591 and Anderson, M. W., et al. in Nature 1994, v367, p. 347-351). ETS-10 has an effective pore size of 8 Å, much larger than the kinetic diameters of ethane and methane, 4.44 Å and 3.76 Å, respectively (see Breck, D. W., in Zeolite Molecular Sieves: Structure, Chemistry and Use; 1974 Wiley-Interscience Publication, John Wiley & Sons, London and Auerbach, S. M., Carrado K. A., in Gas Separation by Zeolites: Handbook of Zeolite Science and Technology, 2003 Marcel Dekker Inc.). As both ethane and methane can enter the crystalline lattice of ETS-10, separation of these species is achieved through equilibrium competitive adsorption.
The Al-Baghli references discussed above are silent to ETS-10 performance at pressures above 1000 kPa (145 psia). Further, the Al-Baghli references actually show that, for a given temperature, the selectivity of ethane/methane separation decreases as the pressure is increased, as is typical for molecular sieve separations, suggesting that these materials may be less suitable for adsorptive separation at higher pressures (see Table 4 of Al-Baghli, N. A. et al. in the Journal of Chemical and Engineering Data 2006, v51, p. 248-254, which shows that at 280K the relative adsorptivity for a binary ethane/methane system decreases from 45.23 at 150 kPa to 30.13 at 500 kPa). Finally, the Al-Baghli references fail to teach the use of cationically or structurally modified variants of the ETS-10 material.
U.S. Pat. Nos. 6,387,159 and 6,521,020 employ a Ca-ETS-10 material to remove hydrocarbons from acid gas streams such as CO2 and especially H2S and discloses adsorption isotherms for propane, butane and pentane, but only at low pressure (i.e. up to 100 Torr). The patents are silent with respect to high pressure removal of ethane from a natural gas stream.
The present invention demonstrates that ETS-10 type materials do, in fact, show good ethane/methane adsorption selectivity at high pressure, including typical operating pressures for natural gas pipelines. We have also found that for some ETS-10 type materials, the ethane/methane selectivity can actually increase as the pressure is increased, which is surprising and unexpected in light of the prior art.